1. Field of the Invention
The present invention relates to a perpendicular magnetic recording medium capable of achieving high-density magnetic recording.
2. Description of the Related Art
For performing high-density magnetic recording, it is necessary to reduce the size of magnetic domains, each of which is a recording unit of information. In the perpendicular magnetic recording medium that is generally used nowadays, information is recorded in a magnetic thin film containing magnetically disrupted magnetic grains. In order to reduce the size of the magnetic domains and to discriminate boundaries thereof for the high-density magnetic recording, it is necessary to reduce the size of magnetic grains.
However, the problem of the thermal fluctuation inhibits the improvement in the recording density of the perpendicular magnetic recording medium. To be more specific, if the magnetic anisotropy energy, which is represented by the product of the magnetic anisotropy energy density Ku and the volume V of a magnetic grain, required to maintain the magnetization of the magnetic grain in a given direction is rendered substantially equal to the thermal fluctuation energy at room temperature, the magnetization is fluctuated with time so as to cause the recorded information to be lost.
For overcoming the thermal fluctuation problem, a patterned media is proposed in, for example, U.S. Pat. No. 5,956,216. The patterned media is prepared by separating a recording layer consisting of a magnetically continuous ferromagnetic thin film into discrete single magnetic domain elements, hereinafter referred to as “ferromagnetic dots”. In this case, it is possible to avoid the thermal fluctuation problem because it is possible to make the volume V of each ferromagnetic dot larger than that of the conventional magnetic grain.
However, processing of the ferromagnetic recording layer is absolutely required for obtaining the patterned media. As a result, it becomes problematic that the magnetic characteristics of the ferromagnetic recording layer are damaged; in particular, the magnitude and the direction of the perpendicular magnetic anisotropy are deteriorated. Also, in order to improve the recording and reproducing efficiency, it is necessary to lower the flying height of the write/read head. However, it is difficult to lower the flying height in the patterned media, which has been irregularly processed, unlike the ordinary medium having a substantially flat surface. Therefore, an additional planarization process is required for lowering the flying height in the patterned media.
It should also be noted that the recording layer itself, i.e., the position of the ferromagnetic dot, determines the position at which the magnetic domain is formed in the patterned media. However, it is difficult to locate the magnetic head accurately at the position of the individual ferromagnetic dot. Where the position of the magnetic pole of the write head deviates greatly from the ferromagnetic dot, though a slight positional deviation is allowable, it is impossible to apply a recording magnetic field to a part of the ferromagnetic dot, with the result that effective recording magnetic field strength is lowered so as to render the recording efficiency poor. Also, if a positional deviation takes place between the read head and the ferromagnetic dot in reproduction, the magnetic flux introduced from the ferromagnetic dot into the read head is decreased so as to lower the reproducing efficiency. Therefore, an appreciably large deviation between the magnetic head and the ferromagnetic dot is not allowable.
As described above, the problem that processing deteriorates the magnetic characteristics of the ferromagnetic recording layer was inherent in the conventional patterned media. Also, the conventional patterned media was defective in that it was difficult to lower the flying height of the magnetic head, and that the allowable positional deviation between the write/read head and the ferromagnetic dot of the recording layer is small.